Recently, a device has been developed for pumping preformed foam down hole in a well to create circulation in the well. This device is described in U.S. Pat. No. 3,463,231, issued Aug. 26, 1969 to Hutchinson, et. al. The foam has been useful in drilling operations, completion operation and maintenance functions, such as sand cleanout.
The foam circulation provides several advantages over more conventional drilling mud circulation. The relatively low hydrostatic head of the foam reduces damage to the oil bearing formation. This also reduces the risk of lost circulation within the formation which can occur with conventional drilling muds if the mud penetrates too far into the formation. The foam has also been found to be quite effective in lifting not only small particles, but relatively large particles having a diameter of several inches.
The preformed foam requires a special mixing aeration generator to create the foam prior to down hole flow. The generator creates foam by flow through porus and flow restricting devices. The generator is of fixed capacity to create a single density of foam. The inability to vary foam density prevents variation in the condition of the foam.
Foam flow requires a greater driving pressure than the pressure needed to flow the individual constituents of the foam. A need exists to control the foam consistency to optimize the foam's action, such as cleanout, at the critical section of the circulation, while minimizing flow constraints in circulation outside the critical region.